Shaft speed monitor

ABSTRACT

An apparatus for monitoring and warning of an undesirable velocity condition in a rotating shaft includes a magnetic reed switch having on and off domains oriented with respect to an actuation magnet which is displaced on an outward radial path with respect to the shaft by centrifugal force, such that the actuation magnet can pass from within an on domain to within an off domain and vice versa, thereby switching the reed switch on and off according to the speed of rotation of the shaft. The reed switch is connected electrically to a warning light or buzzer or control circuit. An alternative embodiment employs a ferromagnetic shield variously interposed between the actuation magnet and the reed switch in response to centrifugal force.

BACKGROUND OF THE INVENTION

The present invention involves a monitoring system which is responsiveto the angular velocity condition of a rotating member of a machine.

One area where such a monitoring system is particularly useful is inagricultural machines, such as seed planters and combines, which mayhave one or more shafts whose continued rotation at a specific speed orwithin a range of speeds is essential to the proper operation andlongevity of the machine.

The approaches certain others have taken to monitor the angular velocityof a shaft are shown in U.S. Pat. Nos. 3,611,343 to Schoenbach;3,739,367 to Fathauer; and 3,921,159 to Steffen. A feature common to thedisclosures of these three patents is the provision of a magneticallyoperable reed switch in proximity with the rotating shaft, and apermanent magnet mounted on the shaft such that the reed switch isactuated by the magnet on each revolution of the shaft. The reed switch,connected to an electrical current source, provides a pulsed DCelectrical signal in which the frequency and duration of the pulsesvaries in accordance with the opening and closing of the reed switch asthe shaft rotates. Electrical circuitry is provided to compare thefrequency or duration of the signal pulses from the reed switch with aselected frequency or duration which corresponds to a desired shaftrotational speed. Deviation from the desired frequency or durationresults in activation of an alarm to indicate to the machine operatorthat an undesirable shaft speed condition is present.

SUMMARY OF THE INVENTION

A shaft speed monitor for warning of an undesired velocity condition ina rotating shaft includes magnetic means for establishing an actuatingmagnetic field and an electrical switch having alternate open and closedstates, the switch being responsive to the actuating magnetic field andcooperating therewith to define on and off spatial domains wherein theswitch is on when the magnetic means is oriented within an on-domain andthe switch is off when the magnetic means is oriented within anoff-domain. Further provided are mounting means for mounting andorienting the magnetic means and the electrical switch with respect tothe shaft to permit displacement by centrifugal force, generated by therotation of the shaft, of the magnetic means with respect to theelectrical switch such that the magnetic means can pass from within anon-domain to within an off-domain. Bias means for biasing the magneticmeans within an on-domain when the shaft is at rest and for resistingdisplacement thereof by centrifugal force are provided. An electricallyoperated output means operatively connected to the electrical switchgenerates an output signal representative of the on-off condition of theswitch.

In another aspect of the present invention, a shaft speed monitor forwarning of an undesired velocity condition in a rotating shaft includesmagnetic means for establishing an actuating magnetic field and anelectrical switch having alternate open and closed states, said switchbeing responsive to the actuating magnetic field. A ferromagnetic sheildis also provided. Mounting means are provided for mounting and orientingsaid magnetic means, said ferromagnetic shield and said electricalswitch with respect to said shaft to permit displacement by centrifugalforce, generated by the rotation of said shaft, of the ferromagneticshield so as to be interposed between the electrical switch and themagnetic means in varying degree to modulate the effect of the actuatingmagnetic field on the electrical switch, thereby causing said switch tochange between the on state and the off state in accordance with thedegree of interposition of the shield. Bias means for biasing saidferromagnetic shield out of interposition when the shaft is at rest andresisting displacement thereof by centrifugal force are provided. Alsoincluded are electrically operated output means operatively connected tosaid electrical switch for generating an output signal representative ofthe on-off condition of said switch.

It is an object of the present invention to provide a simple, reliabledevice for monitoring for an underspeed or overspeed condition in arotating shaft.

Additional objects and advantages will become apparent from thefollowing descriptions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an elevational cross sectional view of a shaft speed monitorapparatus made in accordance with the present invention.

FIG. 2 is a perspective view of an alternative embodiment of a shaftspeed monitor apparatus made in accordance with the present invention.

FIG. 3 is an elevational cross sectional view of yet another embodimentof a shaft speed monitor apparatus made in accordance with the presentinvention.

FIG. 4 is a view of the apparatus of FIG. 3 taken along line 4--4.

FIG. 5 is a front elevational view of another alternative embodiment ofa shaft speed monitor apparatus made in accordance with the presentinvention.

FIG. 6 is a side elevational view of the apparatus of FIG. 5.

DESCRIPTION OF THE PREFERRED EMBODIMENT

For the purposes of promoting an understanding of the present invention,reference will now be made to the embodiment illustrated in the drawingsand specific language will be used to describe the same. It isnevertheless to be understood that no limitation of the scope of theinvention is thereby intended, the proper scope of the invention beingindicated by the claims appended below and the equivalents thereof.

Referring in particular to FIG. 1, there is illustrated an apparatus 10constructed in accordance with the present invention for monitoring thespeed of a rotating shaft, and warning of stoppage or underspeed.Apparatus 10 includes as its principal components a housing 11 mountedwith respect to stationary frame 12 by bearing 13 for rotation aboutaxis 14, a drive shaft 15 coaxial with axis 14 in driving engagementwith housing 11, a magnetic reed switch 16 mounted to frame 12 adjacentthe end face of housing 11 and oriented radially with respect to axis14, an actuation magnet 17 free floating within a cylindrical cavity 18of housing 11, and a retention magnet 19 eccentrically mounted within arecess in the end of drive shaft 15.

In use, apparatus 10 would be located in a machine, such as a farmcombine, adjacent a rotating shaft, the speed of which is desired to bemonitored. Frame 12 would be rigidly mounted to a convenient stationarymember of the machine, with an O-ring or other type drive belt beingreceived about a pulley on the monitored shaft (or alternatively, aboutthe shaft itself) and within annular groove 23 of drive shaft 15. Theratio of the monitored shaft pulley to the drive shaft 15 would beselected for speed reduction or increase as may be appropriate forproper operation of apparatus 10 according to the principles discussedfurther below.

Actuation magnet 17 and retention magnet 19 are found cylindrical"button" permanent magnets having their magnetic poles located on theirfaces and oriented in the same direction such that actuation magnet 17is attracted to retention magnet 19. Consequently, so long as housing 11is at rest, actuation magnet 17 is held against rear wall 24 of cavity18 in alignment with retention magnet 19. Because retention magnet 19 ismounted eccentrically with respect to axis of rotation 14, rotation ofhousing 11 will cause a centrifugal force effect upon similarly offsetactuation magnet 17, tending to force actuation magnet 17 radiallyoutward as the rotational speed of housing 11 increases. This outwardforce is offset by a radially inward force due to the mutual magneticattraction of actuation magnet 17 and retention magnet 19. At someparticular rotational speed, the centrifugal force will overcome themagnetic attraction and actuation magnet 17 will break free and slideoutwardly along rear wall 24. Upon reduction in speed, the magneticattraction will eventually overcome the centrifugal force and actuationmagnet 17 will again be drawn inward into alignment with retentionmagnet 19. The speed at which actuation magnet 17 is drawn back in willgenerally be less than the speed at which it first broke free, thisdiscrepancy being known as hysteresis. To reduce the contribution offriction to this hysteresis, rear wall 24 is made of a low frictionplastic material, preferably Teflon.

Reed switch 16 is the conventionally known type having a pair ofoverlapping metallic ferromagnetic reeds mounted within a hermeticallysealed glass tube, with electrical terminals 25 and 26 at opposite ends.The reeds have the property that when placed within a properly orientedmagnetic field, both reeds are magnetized with opposite polarity, suchthat they are attracted together, making electrical contact with eachother, and completing the electrical circuit between terminals 25 and26. For a particular reed switch and a given orientation of actuatingmagnet, there are three-dimensional "on-domains" and "off-domains"wherein the switch contacts will be closed or open, respectively, uponentry of the actuation magnet within the domains.

In apparatus 10, reed switch 16 is oriented with respect to actuationmagnet 17 such that when actuation magnet 17 is aligned with retentionmagnet 19, it intrudes within an on-domain, and when actuation magnet 17is disposed at the periphery of cavity 18, its circular path carries itthrough a off-domains only. Consequently, reed switch 16 is closedwhenever the rotational speed of housing 11 drops below the speed atwhich actuation magnet 17 is drawn back inward by retention magnet 19.Should the shaft stop completely, actuation magnet 17 returns to rest inalignment with retention magnet 19, within an on-domain. Thus, apparatus10 can be used to warn of an underspeed or stopped shaft, by connectingterminals 25 and 26 in series with an electrical current source and alight, buzzer and/or automatic control device.

It will be noted that the speed at which actuation magnet 17 breaks freeof retention magnet 19 is a function of the eccentric offset ofretention magnet 19, and the strength of the attractive magnetic forcebetween actuation magnet 17 and retention magnet 19. The eccentricoffset must be great enough to provide sufficient centrifugal force atthe speed of interest, yet not so great that the eccentric rotationintermittently takes the actuation magnet out of the on-domain. Inapparatus 10, the eccentricity is fixed, but the magnetic attractiveforce is adjustable by adjusting the distance between the two magnets.Drive shaft 15 can be moved axially with respect to housing 11, with setscrews 27 holding it in a selected position. Inward disposition of driveshaft 15 (and retention magnet 19) results in a greater actuation speedand outward disposition results in a lesser actuation speed. Theretention magnet must not be moved so far away from the actuation magnetthat the retention magnet will not always return the actuation magnet tothe on-domain when the drive shaft is at rest. Graduated markings couldbe provided along drive shaft 15 to facilitate repeated settings ofapparatus 10 to actuate at particularly selected underspeeds. Thehysteresis cannot be readily adjusted in apparatus 10, except as afunction of actuation speed.

Housing 11, drive shaft 15, and the portion of frame 12 which supportsreed switch 16 are made of non-ferromagnetic material, such as aluminumor plastic, so as to neither influence the behavior of actuation magnet17 nor shield reed switch 16 from the magnetic field of actuation magnet17.

Referring to FIG. 2, there is illustrated an alternative embodiment ofthe present invention presenting a variation of the general scheme shownin FIG. 1 in which the eccentric offset of the actuation magnet withrespect to the axis of rotation is adjustable. Apparatus 110 includes ahousing 111 which is essentially a closed rectangular box made of sheetaluminum housing a free floating actuation magnet 117. A retentionmagnet is fixed to the back of housing 111 such that the actuationmagnet 117 is always returned by mutual magnetic attraction intoalignment with the retention magnet 119 regardless of the at-restorientation of housing 111. U-shaped bracket 127 connects housing 111 tothe end face of shaft 115 by means of screw 128. An elongated slot 129in the rear leg of U-shaped bracket 127 permits adjustment of theeccentric offset of retention magnet 119 with respect to the axis ofrotation of shaft 115, thereby allowing adjustment of the speed at whichactuatioin magnet 117 breaks free of retention magnet 119. Reed switch116 is fixed in position with respect to shaft 115, which can be theshaft which is to be monitored, or a separate belt driven shaft as inthe embodiment of FIG. 1.

Referring to FIGS. 3 and 4, there is illustrated yet another embodimentof the present invention which includes features allowing both overspeedand underspeed warning, as well as hysteresis adjustability. Attached toa shaft 215, the speed of which is to be monitored, is a radiallyoriented non-ferromagnetic tube 211 containing an actuation magnet 217with radially oriented magnetic poles. Tube 211 is balanced bycounterweight 231. Outward radial movement of actuation magnet 217caused by centrifugal force is resisted by coil spring 229. Adjustmentscrew 230 provides adjustment of actuation speed by varying the radialposition of coil spring 229, and therefore the spring force acting uponactuation magnet 217 for a given radial displacement of actuation magnet217.

Located radially with respect to shaft 215 are two reed switches 216 and216a having biasing permanent magnets 240 and 241 mounted along the reedswitch centerline opposite the shaft at a position which makes the reedhave a memory function. That is, once a domain is magnetically violatedby the actuation magnet, the reed stays in that electrical state untilthe opposite domain is violated. Biasing magnets 240 and 241 areoriented with oppositely directed polarity so that the resultingon-domains and off-domains of reed switches 216 and 216a are reversedwith respect to each other. Consequently, one reed switch acts as anoverspeed indicator and the other acts as an underspeed indicator. Bothreed switches 216 and 216a are offset axially with respect to tube 211as shown best in FIG. 4. Adjustment of the position of reed switches 216and 216a in the directions indicated by arrows 243 and 244 in FIG. 3provides adjustment of the speed of actuation. Adjustment of theirpositions in the direction indicated by arrow 245 in FIG. 4 providesadjustment of the hysteresis.

With the embodiment shown in FIGS. 3 and 4, there is no provision forindicating sudden stoppage of the shaft in less than one revolutionafter tube 211 has passed over the underspeed indicating reed switch.Indication in such cases could be provided by mounting tube 211 to shaft215 with a ratchet mechanism so that sudden stoppage of shaft 215 wouldresult in at least one continued revolution by tube 211 due to inertia,thereby ensuring that it would pass over the reed switch in anunderspeed state and the on-domain would be violated by the actuationmagnet 217.

By employing permanent magnets (or electromagnets) in various biaspositions in association with magnetically actuated reed switches, it ispossible to manipulate the size and shape and orientation of theon-domains and off-domains associated with the magnetic switch to permitconfigurations of a shaft speed monitor other than as shown in the fewexamples above. One of the primary principles underlying the presentinvention is that a simple shaft speed monitor can be constructed byproviding a permanent magnet which is movable along a radial path withrespect to the rotating shaft, wherein the radial position is a functionof centrifugal force acting upon the magnet caused by rotation of theshaft. A magnetic switch is provided having on-domains and off-domainsoriented with respect to the shaft such that the magnet can pass fromwithin one domain to within the other as it traverses its radial path.

An additional feature of this shaft speed monitor is that it will alsowarn of shaft bearing failure. In the embodiments of FIGS. 1 and 2, asthe monitored shaft bearing deteriorates the shaft will wobble inadversely affecting drive belt tension to the monitor device andeventually give an underspeed warning. In the case of the FIG. 3embodiment, as the monitored shaft bearing deteriorates the shaft wobblewill send the actuation magnet in ever widening circles until eventuallya domain violation will occur which is not of the standard case, thuswarning the operator.

A key principle underlying the present invention is that a shaft speedmonitor can be simply made without complicated electronic circuitry asrequired by previous art by providing a magnetically actuated switchwhich senses an actuating magnetic flux, the strength of which isproportional to centrifugal force generated by the rotating shaft. Manydifferently configured monitors can be made using this concept. Forinstance, the actuating magnetic flux source and the magneticallyactuated switch can be arranged to be physically displaced from oneanother by centrifugal force acting against a biasing force, such asmight be provided by compression of springs, gasses, or other resilientmaterials, or by oppositely oriented magnetic fields. Alternatively, thethe action of the actuating magnetic flux upon the magnetically actuatedswitch can be manipulated by a ferromagnetic shield interposed betweenthe flux source and the switch in varying degree sympathetic tocentrifugal force generated by the rotating shaft. Other means formanipulating the action of a magnetic flux upon a magnetically actuatedswitch in proportion to centrifugal force will be apparent to thoseskilled in the art.

An alternative embodiment applying the above principles is illustratedin FIGS. 5 and 6. Apparatus 310 includes a rotating shaft 311 to whichis mounted a non-ferromagnetic bracket 312 by means of bolt 313. Bracket312 rotates with shaft 311. Bracket 312 includes projecting arm 314 andactuation magnet 315 is rigidly mounted thereto concentric with shaft311. Attached to bracket 312 is tension spring 317, pivot pin 319, andstop pin 320. Ferromagnetic shield 321 is pivotally mounted on pivot pin319, and includes a graduated arm 322 and a moveable weight 323. Shield321 is moveable from the at-rest position shown in FIG. 5, against thebiasing force of spring 317 so as to interpose the shield between magnet315 and reed switch 326, which is mounted on support 325 so as to remainstationary with respect to the shaft. The degree of interposition ofshield 321 is determined by the speed of rotation of shaft 311, whichcauses weighted arm 322 to move outwardly under the influence ofcentrifugal force and pivot shield 321 about pin 319. By moving weight323 along arm 322, the actuation speed of the apparatus 310 can beadjusted.

While the preferred embodiment of the invention has been illustrated anddescribed in some detail in the drawings and foregoing description, itis to be understood that this description is made only by way of exampleto set forth the best mode contemplated of carrying out the inventionand not as a limitation to the scope of the invention which is pointedout in the claims below.

What I claims is:
 1. A shaft speed monitor for warning of an undesiredvelocity condition in a rotating shaft having an axis of rotation,comprising:magnetic means for establishing an actuating magnetic field,the poles of said magnetic means defining an axis of magnetic polarity;an electrical switch having alternate open and closed states, saidswitch being responsive to the actuating magnetic field and cooperatingtherewith to define on and off spatial domains wherein said switch is onwhen said magnetic means is oriented within an on-domain and said switchis off when said magnetic means is oriented within an off-domain;mounting means for mounting and orienting said magnetic means and saidelectrical switch with respect to said shaft to permit displacement bycentrifugal force, generated by the rotation of said shaft, of themagnetic means with respect to the electrical switch such that saidmagnetic means can pass from within an on-domain to within anoff-domain, in which the axis of magnetic polarity of said magneticmeans is parallel to the axis of rotation of said shaft; bias means forbiasing said magnetic means within an on-domain when the shaft is atrest and for resisting displacement thereof by centrifugal force; andelectrically operated output means operatively connected to saidelectrical switch for generating an output signal representative of theon-off condition of said switch.
 2. The shaft speed monitor of claim 1,in which the biasing means includes magnetic retention means havingmutual magnetic attraction with said magnetic means.
 3. The shaft speedmonitor of claim 2, and further including means for adjusting thestrength of mutual magnetic attraction between said magnetic retentionmeans and said magnetic means, thereby providing actuation speedadjustment.
 4. The shaft speed monitor of claim 3, in which the meansfor adjusting includes means for selectively altering the at-restspacing between the magnetic retention means and the magnetic means. 5.The shaft speed monitor of claim 1, in which the biasing means includesresilient spring means.
 6. The shaft speed monitor of claim 5, andfurther including means for adjusting the force of the biasing resilientspring means, thereby providing actuation speed adjustment.
 7. The shaftspeed monitor of claim 2, and further including means for mounting saidmagnetic retention means to be rotatably driven by said shaft about anaxis of rotation, said magnetic retention means being eccentricallymounted with respect to the axis of rotation.
 8. The shaft speed monitorof claim 7, in which said magnetic means is mounted with respect to theaxis of rotation so as to be axially adjacent the magnetic retentionmeans while at rest, and radially displaced therefrom by centrifugalforce generated by eccentric rotation about the axis of rotation whichexceeds the mutual attractive force therebetween.
 9. A shaft speedmonitor for warning of an undesired velocity condition in a rotatingshaft, comprising:magnetic means for establishing an actuating magneticfield; an electrical switch having alternate open and closed states,said switch being responsive to the actuating magnetic field andcooperating therewith to define on and off spatial domains wherein saidswitch is on when said magnetic means is oriented within an on-domainand said switch is off when said magnetic means is oriented within anoff-domain; mounting means for mounting and orienting said magneticmeans and said electrical switch with respect to said shaft to permitdisplacement by centrifugal force, generated by the rotation of saidshaft, of the magnetic means with respect to the electrical switch suchthat said magnetic means can pass from within an on-domain to within anoff-domain; bias means for biasing said magnetic means within anon-domain when the shaft is at rest and for resisting displacementthereof by centrifugal force, including magnetic retention means havingmutual magnetic attraction with said magnetic means; means for adjustingthe strength of mutual magnetic attraction between the magnetic retenionmeans and said magnetic means, thereby providing actuation speedadjustment; and electrically operated output means operatively connectedto said electrical switch for generating an output signal representativeof the on-off condition of said switch.
 10. The shaft speed monitor ofclaim 9, in which the means for adjusting includes means for selectivelyaltering the at-rest spacing between the magnetic retention means andthe magnetic means.
 11. A shaft speed monitor for warning of anundesired velocity condition in a rotating shaft having an axis ofrotation, comprising:magnetic means for establishing an actuatingmagnetic field; an electrical switch having alternate open and closedstates, said switch being responsive to the actuating magnetic field andcooperating therewith to define on and off spatial domains wherein saidswitch is on when said magnetic means is oriented within an on-domainand said switch is off when said magnetic means is oriented within anoff-domain; mounting means for mounting and orienting said magneticmeans and said electrical switch with respect to said shaft to permitdisplacement by centrifugal force, generated by the rotation of saidshaft, of the magnetic means with respect to the electrical switch suchthat said magnetic means can pass from within an on-domain to within anoff-domain; bias means for biasing said magnetic means within anon-domain when the shaft is at rest and for resisting displacementthereof by centrifugal force, including magnetic retention means havingmutual magnetic attraction with said magnetic means; means for mountingsaid magnetic retention means to be rotatably driven by said shaft aboutsaid axis of rotation, said magnetic retention means being eccentricallymounted with respect to said axis of rotation; and electrically operatedoutput means operatively connected to said electrical switch forgenerating an output signal representative of the on-off condition ofsaid switch.
 12. The shaft speed monitor of claim 11, in which saidmagnetic means is mounted with respect to the axis of rotation so as tobe axially adjacent the magnetic retention means while at rest, andradially displaced therefrom by centrifugal force generated by eccentricrotation about the axis of rotation which exceeds the mutual attractiveforce therebetween.